gain altitude and spread out, the odds of downwind deposition are reduced. On the other hand, calm conditions tend to coincide with a condition known as a temperature inversion, where the air is warmer with elevation instead of colder, as is typical. In such a situation, thermal convection will not occur, and finer droplets can hang in the air for long periods of time. This makes them susceptible to light, lateral winds that can carry them long distances or downhill, according to topography. Inversions are more likely to occur between evening and early morning in humid and cloudless conditions. Inversions are of particular concern when applying volatile products that can off-gas into the air long after they are applied. Operators should avoid night-spraying practices that might coincide with an inversion. Understanding the physics of how droplet size, travel speed, distance to target, and environmental conditions affect drift potential empowers a sprayer operator to act accordingly. This sometimes requires compromise, and ultimately (however inconveniently), an operator may have to halt spraying until conditions improve. For more information on spraying best practices, go to www.sprayers101.com and enter the keyword “drift” in the search field. Dr. Jason Deveau is the application technology specialist with the Ministry of Agriculture, Food and Rural Affairs. l ONTARIO GRAIN FARMER 23 JUNE/JULY 2023 Understanding the physics of how droplet size, travel speed, distance to target, and environmental conditions affect drift potential empowers a sprayer operator to act accordingly. some proportion of the spray will always remain airborne, and its fate will depend on atmospheric stability. (See chart below). On warm, sunny days, turbulence will ground airborne droplets or dilute them in a process called atmospheric mixing. As finer droplets Day wind Early morning wind • Turbulence is when the sun heats the ground; the ground heats the air; hot air rises; and cold air sinks. • Turbulence is good • Turbulence mixes the air, dilutes drift, and brings fine droplets down. Day wind moves behind the sprayer and curls under. • Movement is parallel to surface (not turbulent). • When there is no turbulence, there is no mixing. • When there is no mixing, there is no dilution of spray. • Fine droplets released into non-turbulent wind move sideways, therefore do not come down until wind becomes turbulent. Early morning wind moves in a straight line behind the sprayer.
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